This invention relates to the hydraulic fracturing of a subterranean earth formation and more particularly to a method of creating vertical fractures from a horizontal or inclined wellbore.
Hydraulic fracturing techniques have been extensively used for treatment and stimulation of hydrocarbon-bearing formations. Fracturing generally takes place after the interval to be produced is completed and fluid communication between the wellbore and the reservoir is established. Also, wells are sometimes stimulated by fracturing following production decline or significant depletion in a reservoir. In general, communication is established between the wellbore and the subterranean formation, through the casing, by perforations or slots in the casing. Hydraulic fracturing fluid is pumped from surface equipment into the wellbore and through the perforations or slots into the formation zone to be treated or stimulated. Sufficient amount of hydraulic fluid is forced down the wellbore to cause the parting or fracturing of the formation. Fluid flow into the fracture is continued to cause the initiated fracture to propagate further into the formation. Generally, it has been believed that at shallower formation depths the fractures are horizontal relative to the surface when the fluid pressure applied is greater than the overburden pressure in the formation. In deeper formations, when the fluid pressure is sufficiently great enough to cause the formation to fracture the fractures are generally vertical;
The mixing of proppant material with the hydraulic fracturing fluid is widely practiced to maintain the fracture open and allow hydrocarbon fluids within the formation to flow toward the wellbore for production following treatment. Ultimately, fluid communication between the formation and the wellbore through the fracture is governed both by the orientation of the wellbore, and by the in situ stresses present in the formation area of interest. In situ stresses are shown in FIG. 1, and by convention the maximum in situ stress is labeled .sigma..sub.1, and at depth is the vertical overburden stress. The intermediate .sigma..sub.2, and minimum, .sigma..sub.3, stresses would then be oriented in a plane which is perpendicular to the vertical maximum stress plane. Both near wellbore stresses and stresses at a distance from the wellbore, along with fractures naturally existing in the formation will determine and govern the direction of initiation and propagation of fractures during a treatment.
The goal of fracture treatments is the opening and continued exposure of formation faces allowing for increased flow of fluids toward the wellbore in a formation of relatively low permeability. However, prior to the present invention, it was assumed that regardless of the orientation of perforations in the wellbore casing, when hydraulic fracture fluid was pumped into the formation through the perforations, a fracture would initiate in the plane perpendicular to the plane of minimum principal stress. U.S. Pat. No. 3,878,884 to Raleigh is one such patent where the ultimate vertical orientation of fractures is accounted for, but the near well-bore effects and placement of casing perforations are not recognized.
The direction of fracture initiation is not necessarily the direction of ultimate fracture propagation. The fracture growth and orientation are governed by the combined forces of the principal formation stresses prevalent near the wellbore and the fluid dynamics of the pumped hydraulic fluid as it enters the formation. At a distance from the wellbore, formation stresses become controlling, and the fracture will propagate from that point, reorienting if necessary, in a direction perpendicular to the minimum principal in situ stress. FIG. 2 represents the process of fracture reorientation. It is therefore quite possible the induced hydraulic fractures are not planar. Moreover, a reorientation of induced fractures may cause the fracture width to vary, causing initial bridging of proppant material and restricted, ultimate, propped width. With the advent and popularity of horizontal drilling and completions of horizontal or inclined wellbores, patterns of vertical, including multiple vertical fractures have developed. U.S. Pat. No. 4,669,546 to Jennings et al. describes a method of fracturing by placing a single perforation on the low side of a horizontal wellbore to form a single vertical fracture, rather than multiple vertical fractures. U.S. Pat. No. 4,687,061 to Uhri describes a method for creating a first and a second non-parallel fracture from a deviated wellbore.
A method is therefore desired of creating multiple parallel vertical fractures from a horizontal or inclined wellbore which avoids reorientation and associated fracture restriction.